Iliotibial band (ITB) syndrome (ITBS) is the most common cause of lateral knee pain among athletes. [1, 2, 3, 4, 5, 6] ITBS develops as a result of inflammation of the bursa surrounding the ITB and usually affects athletes who are involved in sports that require continuous running or repetitive knee flexion and extension. [1, 2, 3, 7, 8, 9, 10] This condition is, therefore, most common in long-distance runners and cyclists. ITBS may also be observed in athletes who participate in volleyball, tennis, soccer, football, skiing, weight lifting, and aerobics. 
The image below illustrates active stretching of the ITB.
See Football Injuries: Slideshow, a Critical Images slideshow, to help diagnose and treat injuries from a football game that can result in minor to severe complications.
See also Medscape Drugs & Diseases articles Iliotibial Band Friction Syndrome and Physical Medicine and Rehabilitation for Iliotibial Band Syndrome.
See also the Medscape CME & Education topic Medical Interventions Effectively Treat Overuse Injuries in Adult Endurance Athletes.
ITBS is the most common cause of lateral knee pain in runners. Although few studies are available regarding the incidence of ITBS in athletes, some studies cite this condition with an incidence as high as 12% of all running-related injuries.  Several studies of US Marine Corps recruits undergoing basic training determined the incidence of ITBS among this group to vary from 5.3 to 22.2%.
Data are not available regarding the international incidence of ITBS.
The ITB is the condensation of fascia formed by the tensor fascia lata and the gluteus medius and minimus muscles. The ITB is a wide, flat structure that originates at the iliac crest and inserts at the Gerdy tubercle on the lateral aspect of the proximal tibia. This band serves as a ligament between the lateral femoral condyle and the lateral tibia to stabilize the knee. The ITB assists in the following 4 movements of the lower extremity:
Abducts the hip
Contributes to internal rotation of the hip when the hip is flexed to 30°
Assists with knee extension when the knee is in less than 30° of flexion
Assists with knee flexion when the knee is in greater than 30° of flexion
The ITB is not attached to bone as it courses between the Gerdy tubercle and the lateral femoral epicondyle. This lack of attachment allows it to move anteriorly and posteriorly with knee flexion and extension. Some authors hypothesize that this movement may cause the ITB to rub against the lateral femoral condyle, causing inflammation. Other investigators hypothesize that injury of the ITB results from compression of the band against a layer of innervated fat between the ITB and epicondyle. Furthermore, a potential deep space is located under the ITB as it crosses the lateral femoral epicondyle and travels to the Gerdy tubercle. This bursa may become inflamed and cause a clicking sensation as the knee flexes and extends. The inflamed bursa may add another component to ITB tendinitis.
In runners, the posterior edge of the ITB impinges against the lateral epicondyle of the femur just after foot strike in the gait cycle. [7, 8] This friction occurs at or slightly below 30 º of knee flexion. [2, 3, 7] Downhill running and running at slower speeds may exacerbate ITBS as the knee tends to be less flexed at foot strike. [13, 14]
In cyclists, the ITB is pulled anteriorly on the pedaling downstroke and posteriorly on the upstroke. The ITB is predisposed to friction, irritation, and microtrauma during this repetitive movement because its posterior fibers adhere closely to the lateral femoral epicondyle.
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